Seismograph



Dec. 13, 1960 R. H. J. GENEsLAY 2,964,732

, SEISMOGRAPH Filed 0G11. 2, 1958 l INVENTOR RAYMOND H l GE/vEsLAr atent Olice 2,964,732 Patented Dec, 13, 1960 sErsMoGRAPH Raymond Henri Joseph Geneslay, Bourg-la-Reine, France, assignor to Compagnie Generale de Geophyslque, Paris, France, a corporation of France Filed Oct. 2, 1958, Ser. No. 764,865

Claims priority, application France Oct. 12, 1957 11 Claims. (Cl. 340-17) My invention has for its object improvements in seismographs and more particularly in those which are to be used in groups, the different seismographs being secured permanently at spaced points along a cable positioned for instance on the ground or at the bottom of the sea. It is a well known fact that it is essential for such applications to hold the seismograph secured to the cable always in its proper operative position whatever may be the twisting of the cable when laid in position.

There is used generally for this purpose a type of seisrnograph which is, so to speak, self-biasing; in other words, the action of the gravity brings automatically the seismograph into the desired suitable operative condition, whatever may be its angular position round its longitudinal axis which is horizontal.

' Now, according to my invention, I reduce the value of the reluctance of the arm of the magnetic circuit forming the body of the seismograph through which passes a variable flux, this reduction in the reluctance being obtained by a suitably designed shunt circuit.

Attached drawings illustrate an embodiment of my invention. In said drawings:

Fig. 1 is a meridian cross-section of a seismograph of a conventional type incorporating one of the features of my invention.

Figs. 2 and 3 are views of two embodiments of my improved magnetic circuit for such a seismograph.

Fig. 4 illustrates a detail improvement.

Turning to Fig. l, the seismograph is constituted by a body of revolution round the axis 1--2 drawn in dotted lines and located in the plane of the figure. This body of a seismograph of an electromagnetic type includes chiefiy a preferably laminated magnetic circuit including a tubular section 3 and terminal walls 4 and 5. Against the inner surface of the terminal wall 5 of said magnetic circuit is positioned a permanent magnet 6 constituted by a cylindrical member. To this magnet 6 is secured a pole-piece 7 in the shape of a soli-d disc; a movable magnetic member 8 also of revolution round the axis 1--2 is carried by a yielding rod 9 clamped at 10 in the other terminal wall 4. Said movable member S is on the side 11 facing the pole-piece 7 of a semi-spherical shape, while its opposite end forms as shown atubular extension 12 which is located in coaxial registry with a tubular projection 13 formed on the inner surface of the wall 4,'

the registry being actually obtained when the rod 9 carrying the member 8 lies along the axis y1--2.

It is immediately apparent that the magneto-motive force produced by the magnet 6 induces in the central arm of the magnetic circuit constituted by the magnet, the pole-piece 7, the movable member 8 and the tubular projection 13', a predetermined flux. Said arm includes two gaps, respectively between the at surface 22 of the pole-piece 7 facing the member 8 and the semi-spherical surface 11 and between the surfaces facing each other 20 of the tubular extensions 12 and 21 of the projection y13. This produces thus, as readily apparent, a seismograph of a variable reluctance type.

The movable member 8 can practically execute only movements perpendicular to the axis of revolution of the circuit and these movements lead to modifications in the reluctance of the central arm of the magnetic circuit as a consequence of the relative transverse shifting between the axes of the tubular parts 12 and 13.

A winding 16 is fitted either round the permanent magnet 6 or else, as shown in dotted lines, at 16', round the projection 13, whereby the modifications in the magnetic flux obtained through modifications in reluctance induce in said winding voltages which form the output signal of the seismograph.

It is readily apparent that when such a seismograph is arranged horizontally, i.e. when its axis 1-2 is horizontal, the movable member 8 will provide, by reason of its weight, a static flexing of the rod 9 which leads to a biasing of the seismograph, said biasing phenomenon being independent of the angular setting of the seismograph round the axis 1 2, since the seismograph is in the shape of a body of revolution round said axis. In other words, the flexing of the rod 9 leads to a shifting of the axis of the projection 12 and thereby to a modification of the value of the flux passing through the central arm of the circuit, said modifications in the flux being proportional, as a first approximation, to the magnitude of the shifting of the axis of the tubular section 12.

The seismic vibrations which are exerted in a horizontal plane do not modify in practice the shifting of this axis nor consequently the value of the flux and the seismograph is not sensitive to the horizontal components of the seismic vibrations.

In contradistinction, the vertical vibrations produced by the seismic waves lead to modifications in said shifting of the axis of the member 8 and, consequently, to modifications in the flux.

The sensitivity of the seismograph is, for conditions otherwise identical, proportional to the shifting of the axis produced by gravity.

Hitherto, the gap between the terminal surfaces 20 and 21 is of the same magnitude as that between the surfaces 22 and the outer end of the half-sphere 11.

Consequently, and by reason of magnetic attraction, the member 8 is in a non-stable position of equilibrium, so that said member has a tendency to rock, whereby the tubular end 12 of the movable body can come into contact with the tubular projection 13, which would make the seismograph perfectly unfit for proper operation.

On the other hand, the reluctance of the central arm of the magnetic circuit is comparatively large by reason of the fact that the permanent magnet shows, as well known, `a substantial reluctance for the alternating cornponent of the magnetic flux which passes through it when the movable member 8 begins vibrating under the action of seismic waves.

My invention has for its object to cut out these two drawbacks.

According to a primary feature of said invention, the reluctance of the magnetic circuit for the alternating component of the flux is considerably reduced. I obtain this result by surrounding the magnet 6, obviously inside the winding 16 if the latter surrounds said magnet 6, by a magnetic shunt constituted by a mild iron tube provided with a gap designed so as to produce a partial short-circuit of the magnet, i.e. a by-pass for the iiux produced by the magnet which ensures the passage of a flux of the same magnitude as that produced in the central arm, to wit in the member 8.

It is true 'that such an arrangement reduces by one in, i

central arm, but the reduction in sensitivity resulting therefrom is more than compensated 'and in fact to a large extent, by the reduction of the value of the reluctance for the alternating component of'tlie magnetic llux, so that said novel arrangement'leads to a Vclean and substantial gain of sensitivity. TheV sensitivity is for instance readily multiplied by a factor ranging between 5 and-10.

VThis is shown in Fig. 2 which is a partial View ofthe magnetic circuit on a larger scale showing again .a Part of the wall 5, the magnet 6 and the pole-piece 7, together wit-h the winding 16. According to my invention, a magnetic shunt surrounds, as described, the magnet 6 and includes two sections 17 and 18 of a tube made of magnetic metal, said two sections leaving between them a gap 19 which is designed in a manner such that, as already mentioned, the tlux shunted through the tube may be substantially equal to the ux 'passing through the movable member 8. The use of a unity ratio between the shunted and the non-shunted llux ensures an optimum efficiency for the apparatus.

Fig. 3 is a diagram equivalent to` Fig. V2 from a magnetic standpoint and isshows again the magnet 6 with the magnetic circuit 23 including two sections of which one is provided with a gap enclosingV the movable member 8, while the other carries the winding 16 and is provided with a gap 19. The operation of this modification is the same as in the case of Fig. 2.

According to another feature of my invention, I avoid allrisks of tilting and of adherence-for the movable member by giving the gap between 12 and 13,'i.e. the distance between the surfaces 20` and 21 a value which is clearly larger than that between theparts 7 Vand 8, so that the total longitudinal magnetic attraction arising between 7 and 8 may be larger than that obtained between 12 and 13. Thereby, the arrangement formed by the movable member 8 is subjected to -a longitudinal result# ing force of a substantial value which has a tendency t0 draw said member towards the pole-piece 7; thus, the rod 9 is subjected to a tractional stress and no risk. of undesired adherence through tilting is any longer-to befeared as readily shown |by experience.

VA still Vfurther feature of my invention resides in a suitable selection of the relative diameters of the tubular extension 12 on the movable member 8 and the .correspending tubular projection 13 on the wall 4.

Now, I have found that the magnetic attraction ex.- erted between the terminal surfaces 20 and 21 of the two tubular parts includes a component .perpendicular to the axis of the seismograph. Said component .acts after the manner of a spring which has a tendency .to produce a magnetic lack of stability whenever, the axis of the seismograph being horizontal, Vthe rod 9 ilexes under the action .of gravity exerted on the movable member 8. This magnetic componenttacts thus in the same direction as gravity and cooperates thereby in` producing an increase in the biasing, as defined hereinabove, produced by gravity.

In order to obtain a maximum effect for this transverse component, the outer diameter of the projection 13 is selected, according to. the vabove-referred to features of -my invention, so as to be substantially large-rv, than that ofthe tubularrextension 12 of theYp-artS and, consequently, the outer di'ameterof the extension 12 is substantially equal to the inner diameterrof the tubular projection 13. This feature is, in fact, illustratedV in Fig. l of the accompanying drawings.

My invention covers also embodiments which allow executing series of seismograph-s having accurately the same Ifeatures, which is an essential condition for-obtaining reliable deductions starting from the comparison of the indications given by the seismographsassociated along a common cable for a predetermined investigation.

It is apparent that the most important figure in the data of a seismograph is constituted by the value of the natural frequency of the movable member, since this frequency plays a. predominant part in the shape of the response curve of the seismograph as a function of frequency. On the other hand, it is suitable that the different seismographs may all show approximately the same sensitivity.

Now, according to my invention, it is possible to adjust easily to a predeterminedV value these two characteristic values.

Calculation shows and experience proves that the fact of shifting longitudinally the movable member 8 along the axis of revolution modifies the value of the sensitivity. Y

Consequently, means are provided for obtaining this displacement of the movable member in an easy and accurate manner, so as to obtain the desired sensitivity.

On the other hand, the resonance frequency is a function of the operative length of the rod 9, and therefore a further feature of my invention consists in providing an easy adjustment of said operative length of the rod 9.

Fig. 4 illustrates by way of exemplication and by no means in a limiting sens-e one manner or adjusting these two parameters.

The rod 9 frictionally engages an opening 25 provided in the bottom of a tapped and threaded socket member 26. The rod 9 is secured to the end of a second threaded member 2'7 screwed inside said socket member 2.6. The socket member 26 is screwed through its outer thread inside an opening extending through the wall 4 of the seismograph body.

As will be readily ascertained, the screwing of the member 27 into the socket member 26 modifies the operative length of the rod 9 between its front end secured to the movable member 8 and the opening 25 engaged by said rod.

The operative length of the member 9 is thus in a direct relationship with its length measured between its point of attachment with the movable member and the opening 25.

Furthermore, the screwing and unscrewing of the socket 26 inside the wall 4 modifies the position of the member 8 with reference to the magnetic circuit. In other words, it modifies the value of the gap between 12 and 13, on the one hand, and of the gap between 7 and 8, on the other hand, these values varying in opposite directions. v

Consequently, the screwing of the threaded member 27 carrying the rod 9 inside the socket 26 allows adjusting the value of the natural frequency of the apparatus, whereas the screwing of the socket in t-he Vwall 4 allows adjusting its sensitivity.

It will be remarked from this standpoint that the existence of magnetic attraction has an influence on the value of natural frequency, so that the adjustment of the socket 26 has also an influence on the natural frequency and, consequently, the two adjustments `are interdependent. Obviously, it is possible to design different 'arrange-V ments'V for adjusting the values of the natural frequency and yof the sensitivity of the seismograph within the scope of the accompanying claims.

What I claim is: Y Y

1. In a seismograph, the provisionV of a magnetic circuit comprising a permanent magnet having ka given reluctance, 'a pole-piece rigid with said magnet in horizontal coaxial alignment therewith, a flexible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed towards the latter, a member of magnetic material carried by the end of said rod facing the pole-piece in hori- Zontal coaxial alignment therewith and adapted to move in a vertical plane against the elasticity of the flexible rod under the action of gravity and of vertical seismic vibrations, a main magnetic circuit element connecting rigidly the outer end of the magnet with the point of attachment of the flexible rod and a shunt magnetic circuit connecting the two ends of the magnet together independently of the movable member to reduce the reluctance of said permanent magnet.

2. In a seismograph, the provision of a magnetic circuit comprising a `permanent magnet having a given reluctance, a pole-piece rigid with said magnet in horizontal coaxial alignment therewith, a flexible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed towards the lat-ter, a member of magnetic material carried by the end of said rod facing the pole-piece in horizontal coaxial alignment therewith and adapted to move in a vertical plane against the elasticity of the flexible rod under the action of gravity and of vertical seismic vibrations, a main magneticV circuit element connnecting rigidly the outerezend of the magnet with the point of attachment of the exible rod, and a magnetic tube surrounding the magnet for its entire length between its opposite ends to form ashunt circuit across the latter to reduce the reluctance of said permanent magnet, said magnetic tube being provided with a transverse gap the size of which is selected so as to subdivide the ux passing-through the magnet into equal fractions passing through the main circuit and the shunt circuit respectively.

3. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a. magnetic circuit connecting the ends of the magnet and provided with a gap in coaxial horizontal alignment with the magnet, a movable magnetic member adapted to move under the action of gravity and vertical seismic vibrations inside said gap in the circuit with reference to a position in substantial alignment with the horizontal axis of the magnet and a magnetic shunt circuit connecting the two ends of the magnet together independently of the movable member to reduce the reluctance of said permanent magnet.

4. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a magnetic circuit connecting the ends of the magnet and provided with a gap in coaxial horizontal alignment with the magnet, a movable magnetic meme ber adapted to move under the action of gravity and vertical seismic vibrations inside said gap in the circuit with reference to a position in substantial `alignment with the horizontal axis of the magnet and a shunt circuit connecting the ends of the magnet to reduce the reluctance of said permanent magnet and provided with a gap adapted to define a distribution into substantial equal portions of the tlux passing through the magnet between the main circuit and the shunt circuit.

5. A seismograph comprising a permanent magnet having a given reluctance, a magnetic circuit connecting the ends of the magnet and provided with a gap in coaxial horizontal alignment with the magnet, a movable magnetic member adapted to move, under the action of gravity and vertical `seismic vibrations, inside said gap in the circuit with reference to a position in substantial alignment with the horizontal axis of the magnet, -a magnetic shunt circuit connecting the two ends of the magnet together surrounding the entire length thereof independently of the movable member to reduce the reluctance of said permanent magnet, and a winding surrounding the shunt circuit and subjected to the modifications in ux produced by vertical seismic vibrations.

6. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a pole-piece rigid with said magnet in horizontal coaxial alignment therewith, a exible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed towards the latter, a member of magnetic material carried by the end of said -rod facing the pole-piece and adapted to move in a vertical plane against the elasticity ci the exible rod under the action of gravity and of vertical seismic vibrations, a main magnetic circuit element connecting rigidly the outer end of the magnet with the point of attachment of the exible rod, a tubular projection rigid with said main circuit and coaxially surrounding said exible rod up to a distance from the movable member which is clearly larger than the distance separating the movable member from the polepiece on the magnet, and a magnetic shunt circuit connecting the two ends of the magnet together and surrounding the magnet for the entire length thereof independently of the movable member to reduce the reluctance of said permanent magnet.

7. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a pole-piece rigid with said magnet in horinzontal coaxial alignment therewith, a Vflexible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed towards the latter, a member of magnetic material carried by the end of said rod facing the pole-piece in horizontal coaxial alignment therewith and adapted to move in a vertical plane against the elasticity of the flexible rod under the action of gravity and of vertical seismic vibrations, a main magnetic circuit element connecting rigidly the outer end of the magnet with the point of attachment of the exible rod, a tubular projection rigid with said main circuit and coaxially surrounding said exible rod up to a distance from the movable member which is clearly larger than the distance separating the movable member from the pole-piece on the magnet, the inner diameter of said tubular projection being substantially equal to the outer diameter of the movable member facing it, and a shunt magnetic circuit connecting the two ends of the magnet together independently of the movable member and extending over the entire length of said magnet to reduce the reluctance thereof.

8. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a pole-piece rigid with said magnet in horizontal coaxial alignment therewith, a ilexible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed to wards the latter, a member of magnetic material carried by the end of said rod facing the pole-piece and adapted to move in a vertical plane against the elasticity of the ilexible rod under the action of gravity and of vertical seismic vibrations, a main magnetic circuit element connecting rigidly the outer end of the magnet with the point of attachment of the flexible rod, a tubular projection rigid with said main circuit and coaxially surrounding said exible rod up to a distance from the movable member which is clearly larger than the distance separating the movable member from the polepiece on the magnet, the diameter of said tubular projection being clearly larger than the outer diameter of the end of the movable member facing it, and a shunt magnetic circuit connecting the two ends of the magnet together independently of the movable member, said shunt circuit extending over the entire length of said magnet to reduce the reluctance thereof.

9. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a pole-piece rigid with said magnet in hori zontal coaxial alignment therewith, a flexible rod attached to a point in horizontal alignment with the axis of the magnet in registry with the pole-piece and directed towards the latter, a member of magnetic material carried by the end of said rod facing the pole-piece and adapted to move in a vertical plane against the elasticity of the ilexible rod under the action of gravity and of vertical seismic vibrations, means independent of said permanent magnet for adjusting the operative length of the rod to obtain a corresponding modification of the natural frequency of vibration of the rod and movable member system, a main magnet-ic circuit element connecting rigidly the outer end of the magnetv with the point of attachment of the exible rod, and ashuntrmagnetic circuit connecting the two ends of thermagnet together independently ofv thel movable member to' reduce the relucg tance of said permanent magnet. w

' 10. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given reluctance, a magnetic circuit connecting the ends of the magnet and provided with a gap in coaxial horizontal alignment with the magnet, a movable magnetic member adapted to move under the action of gravity and verA tical seismic vibrations inside said gap inthe circuit with reference to a position in substantial alignment with the horizontal axis of the magnet, means independent of said permanent magnet for shifting longitudinally `the movable member along the axis of the magnet to modify the sensitivity of the seismograph, and a magnetic shunt circuit connecting the two ends of the magnet together independently of the movable member to reduce the re' luctance of said permanent magnet.

11. In a seismograph, the provision of a magnetic circuit comprising a permanent magnet having a given re luctance, a pole-piece rigid with said magnet in horizontal coaxial alignment therewith, a main magnetic circuit rigid with the outer end of the magnet, a socket-shaped threaded and tapped member screwed into the main magnetic circuit coaxially with the magnet but independent thereof and in registry with vthe pole-pieceY and provided inv its bottom with a central perforation, a flexible rod frictionally engaging the perforation in said socket and including a head threadedly engaging the Vtapping in the socket, a member of magnetic material carried by the end of said rod facing the pole-piece and adapted to move in a vertical plane against the elasticity of the flexible rod under the action of gravity and of vertical seismic vibrations, the adjustment of the socket in the circuit and of the rod head inthe main circuit providing adjustment of the sensitivity of the scismograph and natural frequency of the rod and movable member system and a shunt magnetic circuit connecting theV two ends of the magnet together independently of the movable member to reduce the reluctance of said permanent magnet.

References Cited in the le of this patent Y UNITD STATES PATENTS 

